1. Field of the Invention
The embodiments discussed herein are related to a method of evaluating a semiconductor device and an apparatus for evaluating a semiconductor device.
2. Description of the Related Art
In recent years, in metal oxide semiconductor field effect transistors (MOSFETs), variation of the threshold voltage at the time of turn-on consequent to the application of voltage to a gate has become a problem. Variation of the threshold voltage leads to problems such as unbalance of electric current flowing in the semiconductor device (the electric current is not balanced) and decreased current efficiency. Therefore, variation of the threshold voltage at the time of turn-on has to be suppressed and in order to suppress this variation, the threshold voltage at the time of turn-on has to be measured accurately.
Among conventional methods of measuring threshold voltage, one proposed method involves applying voltage (gate voltage) to a gate for an arbitrary period, and after application of the voltage to the gate is suspended, measuring the threshold voltage by measuring gate voltage dependence of the current flowing between a source and a drain to calculate the extent to which the threshold voltage varies (for example, see Denais, M., et al, “On-the-fly characterization of NBTI in ultra-thin gate oxide PMOSFET's”, IEEE International Electron Devices Meeting (IEDM) 2004, p. 109-112).
According to another proposed method, alternating current (AC) voltage (gate voltage) such as voltage of a rectangular pulse is applied to a gate for an arbitrary period of time, and after application of the AC voltage to the gate is suspended, the threshold voltage is measured by measuring gate voltage dependence of the current flowing between a source and a drain whereby, the extent to which the threshold voltage varies is calculated (for example, see Japanese Patent Application Laid-Open Publication No. H8-5706).
Nonetheless, with the methods described above, the application of voltage to the gate and measurement of the threshold voltage after application of the voltage to the gate is suspended consume a certain amount of time. Therefore, a problem arises in that the impact of the application of the voltage to the gate relaxes during a period from the suspension of the application of the voltage to the gate until the measurement of the threshold voltage whereby, variation of the threshold voltage is underestimated.
Accordingly, a method of measuring the threshold voltage in a state where a constant voltage is continuously applied to a gate has been proposed (for example, see Sometani, Mitsuru, et al, “Exact Characterization of Threshold Voltage Instability in 4H-SiC MOSFETs by Non-relaxation method”, Materials Science Forum, Vols. 821-823 (2015), pp. 685-688). FIG. 10 is a circuit diagram schematically depicting an apparatus for evaluating a semiconductor device according to a conventional technique described by Sometani, Mitsuru, et al.
The apparatus for evaluating a semiconductor device according to an embodiment depicted in FIG. 10 is an example of an evaluation device that measures variation of a threshold voltage Vth of a MOSFET 11 to evaluate reliability of the MOSFET 11. The apparatus for evaluating a semiconductor device includes the MOSFET 11, which is an n-channel type, for example, and subject to measurement, and a constant-voltage source 12 and a constant-current source 13 that apply electrical stress to the MOSFET 11. A drain of the MOSFET 11 is connected to the constant-current source 13, with a source and body of the MOSFET 11 being grounded. A gate of the MOSFET 11 is connected to a positive terminal of the constant-voltage source 12. A negative terminal of the constant-voltage source 12 is grounded.
FIG. 11 is a characteristic diagram of the voltage applied to the gate of the MOSFET 11 by the constant-voltage source 12 according to the conventional technique. The constant-voltage source 12 has an electromotive force equal to or higher than the threshold voltage Vth of the MOSFET 11, and continuously applies to the gate of the MOSFET 11, a constant gate voltage Vg (≥Vth) equal to or higher than the threshold voltage Vth of the MOSFET 11. The amount of variation ΔVth of the threshold voltage Vth of the MOSFET 11 is found by converting the amount of variation of a source-drain voltage Vsd of the MOSFET 11 measured in a state where a source-drain current Isd of the MOSFET 11 is maintained to be constant.